Conventional container coatings may be derived from a thermally curable formulation that includes particles of a thermoplastic material, typically vinyl chloride polymers (e.g., polyvinylchloride (PVC)), in an organic solvent. When these coatings are applied to a substrate and cured, the thermoplastic can degrade and discolor. To stabilize the thermoplastic material (reduce degradation) during the curing process, epoxy resins such as, for example, polyglycidyl ethers of cyclic polyols like bisphenol A (often referred to as “BADGE”) and epoxy novolacs, may be added to the coating formulation. Epoxy novolacs and epoxidized linseed oil have also been used as stabilizers for thermoplastic coating formulations.
Epoxy resins can be made by reacting monomers such as bisphenol-A (isopropylidene-di-phenol) and epichlorohydrin (1-chloro-2,3-oxypropane) to the intermediate bisphenol-A-diglycidylether (BADGE). This reaction often results in a small amount of unreacted BADGE which, in the food packaging industry, could potentially lead to contamination of the packaged foodstuffs. Another commonly used component in coating compositions is bisphenol-A, often used as a reactive diluent. Undesirably, packaged food can extract these monomers and epoxy functional compounds from the cured coating over time.
To reduce potential contamination of the packaged articles, it is desirable to reduce the extractable epoxy content in the coatings applied to food and beverage containers. Epoxy compounds with high epoxy equivalent weights may be used to reduce extractable epoxy content in the coating. However, if standard epoxies with equivalent weights higher than that of epoxy novolac (e.g., about 178) are incorporated in compositions at an effective level, coating performance is compromised, particularly adhesion. In addition, the hydroxyl functional groups in these high molecular weight compounds react with additives such as secondary film formers (e.g., urea-formaldehyde resins, acrylics, and phenolics) in the coating formulation. This reaction increases crosslinking, which decreases the adhesion and flexibility of the coating. A cured coating that is too brittle (e.g., from too much crosslinking) would be inappropriate for use in deeply drawn metal containers, which require sufficient coating flexibility to bend and form.